High capacity positive electrodes for use in lithium-ion electrochemical cells and methods of making same

ABSTRACT

Positive electrode for lithium-ion electrochemical cells are provided that have capacity retentions of greater than about 95% after 50 charge-discharge cycles when comparing the capacity after cycle 52 with the capacity after cycle 2 when cycled between 2.5 V and 4.7 V vs. Li/Li +  at 30° C. Compositions useful in the provided positive electrodes can have the formula, Li 1+x (Ni a Mn b Co c ) 1−x O 2 , wherein 0.05≦x≦0.10, a+b+c=1, 0.6≦b/a≦1.1, c/(a+b)&lt;0.25, and a, b, and c are all greater than zero. The process of making these positive electrodes includes firing the compositions at 850° C. to 925° C.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a contination of U.S. application Ser. No.14/241,304, filed Feb. 26, 2014, now allowed, which is a national stagefiling under 35 U.S.C. 371 of PCT/US2012/051644, filed Aug. 21, 2012,which claims the benefit of U.S. Provisional Application No. 61/529,307,filed Aug. 31, 2011, the disclosure of which is incorporated byreference in its entirety herein.

FIELD

This disclosure relates to positive electrodes for use in lithium-ionelectrochemical cells.

BACKGROUND

Secondary lithium-ion electrochemical cells typically include a positiveelectrode that contains lithium in the form of a lithium transitionmetal oxide, a negative electrode, a separator, and an electrolyte.Examples of transition metal oxides that have been used for positiveelectrodes include lithium cobalt dioxide and lithium nickel dioxide.Other exemplary lithium transition metal oxide materials that have beenused for positive electrodes include mixtures of cobalt, nickel, and/ormanganese oxides. Negative electrodes typically include graphite,lithium titanates, or alloys comprising electrochemically activeelements such as Si, Sn, Al, Ga, Ge, In, Bi, Pb, Zn, Cd, Hg, and Sb.

The challenges in designing lithium-ion electrochemical cells includeobtaining a balance between high capacity, high charge-discharge rates,low irreversible capacity, cost, and safety. Lithium cobalt oxide(LiCoO₂) is widely used as the positive electrode in lithium-ionelectrochemical cells for use in commercial products such as computersand hand held phones. LiCoO₂ electrodes have high capacity due to thehigh density of LiCoO₂, rapid charge-discharge due to its layeredstructure, and low irreversible capacity. However, LiCoO₂ is expensiveand subject to occasional runaway thermal reactions. To temper theexpense and safety performance, manganese and nickel can be added to theoxide structure forming the so-called NMC (nickel, manganese, cobalt)oxides of lower cost and higher stability. However, the capacity ofthese oxides has not increased substantially over the capacity ofLiCoO₂.

Typically, layered lithium transition metal oxides are never fullydelithiated during cycling (charging) due to structural instability ofthe fully delithiated state. Thus, one way to achieve higher capacitiesis to design positive electrode materials that have increased stabilityat higher delithiation and, therefore, can be cycled to higher voltages(e.g., 4.3 to 4.8 V vs. Li/Li⁺ or greater). Typical lithium mixed metaloxide (NMC) positive electrode materials, such asLi[Ni_(0.42)Mn_(0.42)Co_(0.16)]O₂, will cycle well up to a voltagecharge of about 4.35 V to 4.4 V (vs. Li/Li⁺). However, when suchmaterials are charged beyond this value, e.g., to 4.8 V vs. Li/Li⁺, thecycle life is quite poor.

SUMMARY

There is a need for positive electrode materials that are useful inlithium-ion electrochemical cells that can allow cycling to highvoltages (e.g., 4.8 V vs. Li/Li⁺) with little capacity fade from cycleto cycle. The provided positive electrode materials, electrodes madethere from, and methods of making the same materials can allow highvoltage cycling with little or reduced capacity fade from cycle to cyclecompared to conventional materials.

In one aspect, a positive electrode for a lithium-ion electrochemicalcell is provided that includes a composition having the formula,Li_(1+x)(Ni_(a)Mn_(b)Co_(c))_(1−x)O₂, wherein 0.05≦x≦0.10, a+b+c=1,0.6≦b/a≦1.1, c/(a+b)<0.25, a, b, and c are all greater than zero andwherein said composition has a capacity retention of greater than about95% after 50 cycles when comparing the capacity after cycle 52 with thecapacity after cycle 2 when cycled between 2.5 V and 4.7 V vs. Li/Li⁺ at30° C. The composition can be prepared by firing precursors to atemperature ranging of from 850° C. to 925° C. The amount of cobalt canvary from about 10 to about 20 molar percent (based on total metalcontent) and the amount of manganese and nickel can be about the same(b/a is about 1). The amount of excess lithium can vary from 5 to 7%(0.05≦x≦0.07). Similar compositions can have greater than about 90%capacity retention after 52 cycles compared to the capacity after cycle2 when cycled between 2.5 V and 4.7 V vs. Li/Li⁺ at 50° C.

In another aspect, a positive electrode for a lithium-ionelectrochemical cell is provided that includes a composition thatcomprises a plurality of particles having a core having the formula,Li_(1+x)(Ni_(a)Mn_(b)Co_(c))_(1−x)O₂, wherein 0.05≦x≦0.10, a+b+c=1,0.6≦b/a≦1.1, c/(a+b)<0.25, a, b, and c are all greater than zero and ashell substantially surrounding the core, the shell comprising a lithiummixed transition metal oxide comprising manganese and nickel wherein themolar ratio of manganese to nickel is greater than 1, wherein saidcomposition has a capacity retention of greater than about 95% after 52cycles compared to the capacity after cycle 2 when cycled between 2.5 Vand 4.7 V vs. Li/Li⁺ at 30° C.

In yet another aspect, a method of making a positive electrode havingthe formula, Li_(1+x)(Ni_(a)Mn_(b)Co_(c))_(1−x)O₂,wherein 0.05≦x≦0.10,a+b+c=1, 0.6≦b/a≦1.1, c/(a+b)<0.25, a, b, and c are all greater thanzero for a lithium-ion electrochemical cell is provided that includesfirst forming a mixed metal hydroxide or carbonate by precipitating anaqueous mixture of Ni:Mn:Co salts in a molar ratio of a:b:c with ahydroxide or carbonate source, drying the mixed metal hydroxide orcarbonate, mixing the mixed metal hydroxide or carbonate with a Lisource to provide a molar ratio of Li to transition metals of[(1+x)/(1−x)] to 1, sintering the mixture at about 500° C. for at leastabout 4 hours, and firing the mixture at from about 850° C. to about925° C. for at least 12 hours after sintering.

Finally, in another aspect, a method of making a positive electrode isprovided that includes first forming a mixed metal hydroxide orcarbonate by precipitating an aqueous mixture of Ni:Mn:Co salts with ahydroxide or carbonate source, where the molar ratio a:b:c is withrespect to the formula, Li_(1+x)(Ni_(a)Mn_(b)Co_(c))_(1−x)O₂, wherein0.05≦x≦0.10, a+b+c=1, 0.6≦b/a≦1.1, and c/(a+b)<0.25, a, b, and c are allgreater than zero, drying the hydroxide or carbonate to form a drytransition metal hydroxide or carbonate powder, dispersing this powderin ammoniated water; as a seed dispersion, heating the mixture to about60° C.; adding an aqueous solution of soluble mixed transition metalsalts comprising manganese and nickel wherein the molar ratio ofmanganese to nickel is greater than 1, and precipitating a hydroxide orcarbonate shell onto the core particles to form a core-shel hydroxide orcarbonate; drying the core-shell hydroxide or carbonate; mixing thecore-shell hydroxide or carbonate with a lithium source (e.g. LiOH, orLi₂CO₃ salt); sintering the mixture at about 500° C. for at least about4 hours; and firing the mixture at from about 850° C. to about 925° C.for at least 12 hours after sintering.

In the present disclosure:

“cycling” refers to lithiation followed by delithiation or vice versa;

“negative electrode” refers to an electrode (often called an anode)where electrochemical oxidation and delithiation occurs during adischarging process; and

“positive electrode” refers to an electrode (often called a cathode)where electrochemical reduction and lithiation occurs during adischarging process; and

“substantially surrounding” refers to a shell that almost completelysurrounds the core, but may have some imperfections which expose verysmall portions of the core.

The provided positive electrodes and electrochemical cell containingthese positive electrodes can be operated at high voltages (greater than4.4 V vs. Li/Li⁺ and up to about 4.8 V vs. Li/Li⁺) with higher retentionof capacity after multiple cycles than conventional electrodes andcells.

The above summary is not intended to describe each disclosed embodimentof every implementation of the present invention. The brief descriptionof the drawings and the detailed description which follows moreparticularly exemplify illustrative embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a comparative graph of capacity (mAh/g) vs. cycle number forthe composition of Example 1 and a competitive material (commercialLi_(1+x)(Mn_(0.42)Ni_(0.42)Co_(0.16))_(1−x)O₂, (Commercial Material Awith x=0.03-0.04) fired at 1000° C.) cycled at 30° C. and at 50° C.

FIG. 2 is a graph of cell voltage (V) vs. capacity (mAh/g) for thecomposition of Example 1 cycled at 50° C.

FIG. 2a is a graph of the differential capacity dQ/dV vs V for thevoltage curve of FIG. 2.

FIG. 3 is a graph of cell voltage (V) vs. capacity (mAh/g) for thecomposition of Comparative Example 1 cycled at 50° C.

FIG. 3a is a graph of the differential capacity dQ/dV vs V for thevoltage curve of FIG. 3.

FIG. 4 is a comparative graph of capacity (mAh/g) vs. cycle number forcompositions of Example 2 and Comparative Example 1 cycled at 50° C.

FIG. 5 is a comparative graph of capacity (mAh/g) vs. cycle number forcompositions of Example 3 and Comparative Example 3 material cycled at50° C.

FIG. 6 is a comparative graph of capacity (mAh/g) vs. cycle number forcompositions of Comparative Example 4 cycled at 50° C.

FIG. 7 is a graph of capacity (mAh/g) vs. cycle number for compositionsof Example 1, Example 6, and commercialLi_(1.05)[Ni_(0.42)Mn_(0.42)Co_(0.16)]O₂ (Commercial Material A) cycledat 50° C.

DETAILED DESCRIPTION

In the following description, reference is made to the accompanying setof drawings that form a part of the description hereof and in which areshown by way of illustration several specific embodiments. It is to beunderstood that other embodiments are contemplated and may be madewithout departing from the scope or spirit of the present invention. Thefollowing detailed description, therefore, is not to be taken in alimiting sense.

Unless otherwise indicated, all numbers expressing feature sizes,amounts, and physical properties used in the specification and claimsare to be understood as being modified in all instances by the term“about.” Accordingly, unless indicated to the contrary, the numericalparameters set forth in the foregoing specification and attached claimsare approximations that can vary depending upon the desired propertiessought to be obtained by those skilled in the art utilizing theteachings disclosed herein. The use of numerical ranges by endpointsincludes all numbers within that range (e.g. 1 to 5 includes 1, 1.5, 2,2.75, 3, 3.80, 4, and 5) and any range within that range.

Positive electrode materials have been developed for lithium-ionelectrochemical cells that are less expensive and safer than LiCoO₂.Mixed lithium transition metal oxides that include nickel, manganese,and cobalt (NMC) are replacing lithium cobalt oxide increasingly incommercial batteries. For example, formulations such asLi_(1+x)(Ni_(0.42)Mn_(0.42)Co_(0.16))_(1−x)O₂ (herein referred to asCommercial Material A), andLi_(1+x)[(Ni_(1/3)Mn_(1/3)Co_(1/3))_(1−x)]O₂, (hereinafter referred toas Commercial Material B) are available commercially (for example, from3M Company, St. Paul, Minn.). These materials may have excess lithium ofabout 3-4 percent (x is about 0.03-0.04), can be fired at a hightemperature (usually around 1000° C.) and are optimized for cycling inrechargeable cells in the range of 2.5 V to about 4.3 V vs. Li/Li⁺. Ifthese materials are charged to a high voltage, such as, for example, 4.8V, they do not show strong signs of an oxygen loss plateau around 4.6 Vand the capacity retention during cycling is low (about 25% capacityloss after about 50 cycles and almost complete loss of capacityretention after 150 cycles).

Kang et al. in “Study of Li_(1+x)(Mn_(4/9)Co_(1/9)Ni_(4/9))_(1−x)O₂Cathode Materials for Vehicle Battery Applications”, J. ElectrochemicalSociety, 158 (8) A936-A941 (2011), describe a NMC type oxide cathodematerial Li_(1+x)(Mn_(4/9)Co_(1/9)Ni_(4/9))_(1−x)O₂ that similarlydisplays weak “oxygen loss” character, when initially cycled to 4.6 V.As with the above mentioned NMC type materials, the authors point outthat their material cycles well between 2.5 V to 4.4 V but suffersconsiderable capacity loss with cycling when oxidized to highervoltages. Surprisingly, we have found that by slightly increasing theexcess lithium, and by controlling the oxide firing temperature, NMCtype oxides can be produced that have greatly improved cycle lifeperformance when cycled to 4.7 V vs. Li/Li⁺ not only at ambienttemperature (30° C.) but also at elevated temperature (50° C.).

As is well known to those of ordinary skill in the art, lithium mixedmetal oxide materials can exist in a layered O3 structure withalternating layers of transition metal oxide and lithium each layerhaving an oxygen layer in between. The O3 structure allows relativelyfacile removal and insertion of a fraction of the lithium without muchchange in structure over a voltage range from about 2.5 to 4.3 V.Accordingly, the presence of an O3 layered structure enables the abilityto charge, discharge, and recharge the material (by readily moving afraction of lithium ions and electrons in and out of the lithium layerin the crystal structure) multiple times without much capacity loss. Theaverage oxidation state of the transition metal layer is correspondinglyraised or lowered.

Certain oxide compositions allow the incorporation of additional Li(excess Li) into the layered structure. This allows for the formation ofa solid state solution of LiMO₂ and Li₂MnO₃. When such a virgin lithiummixed metal oxide material is first charged, lithium ions (andelectrons) are removed from the layered structure. If the voltage israised high enough and the transition metal layer has reach its highestoxidation state i.e., greater than about 4.6 V vs. Li/Li⁺, electrons canstill be forced to leave the layered structure at the irreversibleexpense of oxygen. At these higher voltages this is known as “oxygenloss”. Commercial NMC materials do not show a strong oxygen losscharacter when taken to 4.8V, and if cycled above 4.4V display poorcapacity retention.

Surprisingly, when some lithium mixed metal oxides (NMC oxides) havebetween 5 and 10 percent excess lithium and are prepared by firing at anarrow temperature range of 850° C. to 925° C., materials with improvedcycling at high voltages can be produced. It has additionally been foundthat this improved performance is not universal, but compositiondependent. NMC oxides only display this cycling improvement when themolar ratio of cobalt to the sum of the remaining transition metal isless than 25% and the molar ratio of Mn to Ni is between 1.1 to 0.6.Materials of the formula, Li_(1+x)[(Ni_(a)Mn_(b)Co_(c))_(1−x)]O₂,wherein a+b+c=1, b/c=0.6 to 1.1 and x=0.05 to 0.1 meet this requirement.

Positive electrodes for a lithium-ion electrochemical cell are providedthat include a composition having the formulaLi_(1+x)(Ni_(a)Mn_(b)Co_(c))_(1−x)O₂ wherein 0.05≦x≦0.10, a+b+c=1,0.6≦b/a≦1.1, and c/(a+b)<0.25. Additionally, these electrodes arecharacterized in that the composition has a capacity retention ofgreater than about 95% after 50 cycles comparing the capacity aftercycle 2 to the capacity after cycle 52 when cycled between 2.5 V and 4.7V vs. Li/Li⁺ at 30° C. In some embodiments, 0.10≦c≦0.20. In otherembodiments, the ratio of b to a or b/a is about 1. In some embodiments,0.05≦x≦0.07. In some embodiments, said composition has a capacityretention of greater than about 90% after 50 cycles comparing thecapacity after cycle 2 to the capacity after cycle 52 when cycledbetween 2.5 V and 4.7 V vs. Li/Li⁺ at 50° C.

In some embodiments, a positive electrode for a lithium-ionelectrochemical cell comprising a composition that comprises a pluralityof particles having a core and being substantially surrounded by ashell, the core having the formula,Li_(1+x)(Ni_(a)Mn_(b)Co_(c))_(1−x)O₂, wherein 0.05≦x≦0.10, a+b+c=1,0.6≦b/a≦1.1, c/(a+b)<0.25, and the shell comprising a lithium mixedtransition metal oxide comprising manganese and nickel wherein the molarratio of manganese to nickel is greater than >1, wherein saidcomposition has a capacity retention of greater than about 95% after 50cycles recorded at the same rate cycled between 2.5 V and 4.7 V vs.Li/Li⁺ at 30° C. Additionally, these electrodes are characterized inthat the composition has a capacity retention of greater than about 95%after 50 cycles comparing the capacity after cycle 2 to the capacityafter cycle 52 when cycled between 2.5 V and 4.7 V vs. Li/Li⁺ at 30° C.In some embodiments, 0.10≦c≦0.20. In other embodiments, the ratio of bto a or b/a is about 1. In some embodiments, 0.05≦x≦0.07. In someembodiments, said composition has a capacity retention of greater thanabout 90% after 50 cycles recorded at the same rate cycled between 2.5 Vand 4.7 V vs. Li/Li⁺ at 50° C.

The provided positive electrodes can be incorporated into a lithium-ionelectrochemical cell. The provided lithium-ion electrochemical cells mayalso include a negative electrode comprising carbon, graphite,titanates, alloys or mixtures of these. Useful alloy active materialsinclude silicon, tin, or a combination thereof. Additionally the alloysinclude at least one transition metal. Suitable transition metalsinclude, but are not limited to, titanium, vanadium, chromium,manganese, iron, cobalt, nickel, copper, zirconium, niobium, molybdenum,tungsten, and combinations thereof. The alloy active materials can also,optionally, include silver, lead, germanium, phosphorus, gallium,bismuth, aluminum, indium, carbon, or one or more of yttrium, alanthanide element, an actinide element or combinations thereof.Suitable lanthanide elements include lanthanum, cerium, praseodymium,neodymium, promethium, samarium, europium, gadolinium, terbium,dysprosium, holmium, erbium, thulium, ytterbium, and lutetium. Suitableactinide elements include thorium, actinium, and protactinium. Somealloy compositions contain a lanthanide elements selected, for example,from cerium, lanthanum, praseodymium, neodymium, or a combinationthereof.

Typical alloy active materials can include greater than 55 mole percentsilicon. Useful alloy active materials can be selected from materialsthat have the following components, SiAlFeTiSnMm, SiFeSn, SiAlFe, SnCoC,and combinations thereof where “Mm” refers to a mischmetal thatcomprises lanthanide elements. Some mischmetals contain, for example, 45to 60 weight percent cerium, 20 to 45 weight percent lanthanum, 1 to 10weight percent praseodymium and, 1 to 25 weight percent neodymium. Othermischmetals contains 30 to 40 weight percent lanthanum, 60 to 70 weightpercent cerium, less than 1 weight percent praseodymium, and less than 1weight percent neodymium. Still other mischmetals contains 40 to 60weight percent cerium and, 40 to 60 weight percent lanthanum. Themischmetal often includes small impurities (e.g., less than 1 weightpercent, less than 0.5 weight percent, or less than 0.1 weight percent)such as, for example, iron, magnesium, silicon, molybdenum, zinc,calcium, copper, chromium, lead, titanium, manganese, carbon, sulfur,and phosphorous. The mischmetal often has a lanthanide content of atleast 97 weight percent, at least 98 weight percent, or at least 99weight percent. One exemplary mischmetal that is commercially availablefrom Alfa Aesar, Ward Hill, Mass. with 99.9 weight percent puritycontains approximately 50 weight percent cerium, 18 weight percentneodymium, 6 weight percent praseodymium, 22 weight percent lanthanum,and 3 weight percent other rare earths.

Exemplary active alloy materials include Si₆₀Al₁₄Fe₈TiSn₇Mm₁₀,Si₇₁Fe₂₅Sn₄, Si₅₇Al₂₈Fe₁₅, Sn₃₀Co₃₀C₄₀, or combinations thereof. Theactive alloy materials can be a mixture of an amorphous phase thatincludes silicon and a nanocrystalline phase that includes anintermetallic compound that comprises tin. Exemplary alloy activematerials useful in the provided lithium-ion electrochemical cells canbe found, for example, in U.S. Pat. No. 6,680,145 (Obrovac et al.), U.S.Pat. No. 6,699,336 (Turner et al.), and U.S. Pat. No. 7,498,100(Christensen et al.) as well as in U. S. Pat. Publ. Nos. 2007/0148544(Le), 2007/0128517 (Christensen et al.), 2007/0020522, and 2007/0020528(both Obrovac et al.).

Provided electrochemical cells require an electrolyte. A variety ofelectrolytes can be employed. Representative electrolytes can containone or more lithium salts and a charge-carrying medium in the form of asolid, liquid or gel. Exemplary lithium salts are stable in theelectrochemical window and temperature range (e.g. from about −30° C. toabout 70° C.) within which the cell electrodes can operate, are solublein the chosen charge-carrying media, and perform well in the chosenlithium-ion cell. Exemplary lithium salts include LiPF₆, LiBF₄, LiClO₄,lithium bis(oxalato)borate, LiN(CF₃SO₂)₂, LiN(C₂F₅SO₂)₂, LiAsF₆,LiC(CF₃SO₂)₃, and combinations thereof. Exemplary solid electrolytesinclude polymeric media such as polyethylene oxide, fluorine-containingcopolymers, polyacrylonitrile, combinations thereof and other solidmedia that will be familiar to those skilled in the art. Exemplaryliquid electrolytes include ethylene carbonate, propylene carbonate,dimethyl carbonate, diethyl carbonate, ethyl-methyl carbonate, butylenecarbonate, vinylene carbonate, fluoroethylene carbonate, fluoropropylenecarbonate, γ-butyrolactone, methyl difluoroacetate, ethyldifluoroacetate, dimethoxyethane, diglyme (bis(2-methoxyethyl)ether),tetrahydrofuran, dioxolane, combinations thereof and other media thatwill be familiar to those skilled in the art. Exemplary electrolyte gelsinclude those described in U.S. Pat. No. 6,387,570 (Nakamura et al.) andU.S. Pat. No. 6,780,544 (Noh).

The electrolyte can include other additives that will be familiar tothose skilled in the art. For example, the electrolyte can contain aredox chemical shuttle such as those described in U.S. Pat. Appl. Publ.No. 2009/0286162 (Lamanna et al.).

Composite electrodes, such as the provided positive electrodes, cancontain additives such as will be familiar to those skilled in the art.The electrode composition can include an electrically conductive diluentto facilitate electron transfer between the composite electrodeparticles and from the composite to a current collector. Electricallyconductive diluents can include, but are not limited to, carbon black,metal, metal nitrides, metal carbides, metal silicides, and metalborides. Representative electrically conductive carbon diluents includecarbon blacks such as SUPER P and SUPER S (both from MMM Carbon,Belgium), SHAWANIGAN BLACK (Chevron Chemical Co., Houston, Tex.),acetylene black, furnace black, lamp black, graphite, carbon fibers andcombinations thereof.

The electrode composition can include an adhesion promoter that promotesadhesion of the composition and/or electrically conductive diluent tothe binder. The combination of an adhesion promoter and binder can helpthe electrode composition better accommodate volume changes that canoccur in the composition during repeated lithiation/delithiation cycles.Alternatively, the binders themselves can offer sufficiently goodadhesion to metals and alloys so that addition of an adhesion promotermay not be needed. If used, an adhesion promoter can be made a part ofthe binder itself (e.g., in the form of an added functional group), canbe a coating on the composite particles, can be added to theelectrically conductive diluent, or can be a combination of suchmeasures. Examples of adhesion promoters include silanes, titanates, andphosphonates as described in U.S. Pat. Appl. Publ. No. 2004/0058240 A1(Christensen).

A method of making a positive electrode having the formulaLi_(1+x)(Ni_(a)Mn_(b)Co_(c))_(1−x)O₂ is provided, wherein 0.05≦x≦0.10,a+b+c=1, 0.6≦b/a≦1.1, c/(a+b)<0.25 for a lithium-ion electrochemicalcell comprising: combining first forming a mixed metal hydroxide orcarbonate by precipitating an aqueous mixture of Ni:Mn:Co salts in amolar ratio of a:b:c with a hydroxide or carbonate source, drying themixed metal hydroxide or carbonate, then mixing the mixed metalhydroxide or carbonate with a Li source to provide a molar ratio of Lito transition metals of [(1+x)/(1−x)] to 1. Sintering the mixture atabout 500° C. for at least about 4 hours; and firing the mixture at fromabout 850° C. to about 925° C. for at least 12 hours after sintering. Insome embodiments, 0.10≦c≦0.20. In other embodiments, the ratio of b to aor b/a is about 1. In some embodiments, 0.05≦x≦0.07. In someembodiments, said composition has a capacity retention of greater thanabout 90% after 50 cycles comparing the capacity after cycle 52 to thecapacity after cycle 2 when cycled between 2.5 V and 4.7 V vs. Li/Li⁺ at50° C.

Finally, a method of making a positive electrode is provided thatincludes first forming a mixed metal hydroxide or carbonate byprecipitating an aqueous mixture of Ni:Mn:Co salts in a molar ratio ofa:b:c with a hydroxide or carbonate source, drying the mixed metalhydroxide or carbonate, dispersing the powder in ammoniated water;heating the mixture to about 60° C.; adding an aqueous solution ofsoluble mixed transition metal salts comprising manganese and nickelwherein the molar ratio of manganese to nickel is greater than b/a andgreater than 1 to the hydroxide or carbonate dispersion andprecipitating with a hydroxide or carbonate source to form a core-shellhydroxide or carbonate; drying the core-shell hydroxide or carbonate;mixing the core-shell hydroxide or carbonate with a lithium salt toprovide a Li to transition metal (combined core and shell) molar ratioof [(1+x)/(1−x)] to 1; sintering the mixture at about 500° C. for atleast about 4 hours; and firing the mixture at from about 850° C. toabout 925° C. for at least 12 hours after sintering.

Objects and advantages of this invention are further illustrated by thefollowing examples, but the particular materials and amounts thereofrecited in these examples, as well as other conditions and details,should not be construed to unduly limit this invention.

EXAMPLES Cycling Experiments

Positive electrode compositions were prepared by mixing the oxide,conductive diluent (Super P, MMM), and binder (PVDF, Aldrich Chemicals)in weight composition of (90:5:5) in MNP (Aldrich Chemicals) using aMazerustar Mixer for 8 minutes. The suspension was coated onto Aluminumfoil with a notch bar coater, and the coating dried in an oven at 100°C. The coating was cut into electrodes and assembled into 2325 coincells using a metallic Li counter electrode. The cells were cycled usinga Maccor Cycler (Maccor, Tulsa, Okla.) by charging and dischargingbetween 2.5V and 4.8V vs. Li/Li⁺ for the first two cycles at a chargeand discharge rate of C/10., then between 2.5 and 4.7V for the followingcycles, at a charge and discharge rate of C/4. The cells were heldequilibrated in an oven at 30° C. or 50° C., the capacity retention over50 cycles was determined by comparing the capacity after cycle 52 to thecapacity after cycle 2.

Example 1

15 g of a mixed transition metal hydroxide of the composition(Ni_(0.42)Mn_(0.42)Co_(0.16))(OH)₂ (available from OMG, Kokkola,Finland) was mixed with 7.8 g Li(OH).H₂O in a mortar and the mixturesintered at 500° C. for 4 hrs, then fired at 900° C. for 12 hrs to formLi_(1+x)[(Ni_(0.42)Mn_(0.42)Co_(0.16))_(1−x)]O₂, with x=0.06.

FIG. 1 shows a graph of capacity (mAh/g) vs. cycle number for thecomposition of Example 1 compared to Commercial Material A. CommercialMaterial A is a lithium mixed metal oxide having the formula,Li_(1+x)[(Ni_(0.42)Mn_(0.42)Co_(0.26))_(1−x)]O₂, with x about 0.03-0.04.The cycling was done as described above at 30° C. and at 50° C. for bothmaterials. The graph shows less capacity loss during cycling for Example1 at both 30° C. and 50° C. compared to Commercial material A of similartransition metal composition. The composition of Example 1 has moreexcess lithium than Commercial Material A and was fired at a lowertemperature than that of Commercial Material A. The results show that atboth cycling temperatures, the slope of the change in capacity as afunction of cycle number is less for the composition of Example 1 thanfor Commercial Material A.

FIG. 2 is a graph of cell voltage (V) vs. capacity (mAh/g) for thecomposition of Example 1. An “oxygen loss” plateau can be observed inFIG. 2 at around 4.5-4.7 volts. The existence of an “oxygen loss”plateau is particularly clear by observing the differential capacityplot (dQ/dV vs Q) showing a distinct peak at ˜4.6 V (see FIG. 2a )

FIG. 3 is a graph of cell voltage (V) vs. capacity (mAh/g) forCommercial Material A. This graph does not show much, if any, of an“oxygen loss” plateau (lack of peak around 4.6V in the dQ/dV vs Q plot(see FIG. 3a ).

Example 2

The hydroxide (Ni_(0.5)Mn_(0.4)Co_(0.1))(OH)₂ was produced in a 500 mLstirred tank reactor equipped with temperature control, controlledstirrer speed, and pH probe control. A 2M solution of Ni(SO₄).H₂O,Mn(SO₄).H₂O, and Co(SO₄).H₂O with a molar ratio of 5:4:1 Ni:Mn:Co wasmetered into the reactor containing 75 mL of distilled water at a rateof 2-5 mL/min. During the reaction, the reactor pH was maintained at9.9-10.0, the temperature was ambient and the solution was notdeaerated. Addition of the sulfate solution continued till the maximumvolume of the reactor was reached. The hydroxide was then washed anddried to form a transition metal hydroxide

15 g of this hydroxide was mixed with 7.8 g Li(OH).H₂O in a mortar andthe mixture sintered at 500° C. for 4 hrs, then fired at 900° C. for 12hrs to form a target composition ofLi_(1+x)(Ni_(0.5)Mn_(0.4)Co_(0.1))_((1−x))O₂ with x=0.06.

Comparative Example 1

In this example the oxide Li_(1+x)[(Ni_(0.5)Mn_(0.4)Co_(0.1))_((1−x))]O₂was produced with x=0.05 and fired at 1000° C.

6 g of the hydroxide described above was mixed with 3.04 g Li(OH).H₂O ina mortar and the mixture sintered at 500° C. for 4 hrs, then fired at1000° C. for 12 hrs to form a target composition ofLi_(1+x)(Ni_(0.5)Mn_(0.4)Co_(0.1))_((1−x))O₂ with x=0.05.

FIG. 4 is a comparative graph of capacity (mAh/g) vs. cycle number forcompositions of Example 2 and Comparative Example 1. Both compositionshave mixed metal oxides with 50% nickel, 40% manganese, and 10% cobalt.The composition of Comparative Example 1 has 5% excess lithium (x=0.05)and the composition of Example 2 has 6% excess lithium. FIG. 4 shows thedifference in capacity as a function of cycling at 50° C. for the samecompositions except that Example 2 has more excess lithium thatComparative Example 1 and the composition of Example 1 was fired at atemperature of 850° C. to 950° C. instead of 1000° C.

Comparative Example 2

In this example we compare commercial Li(Ni_(1/3)Mn_(1/3)Co_(1/3))O₂(BC-618, available from 3M company, St. Paul, Minn.) withLi_(1+x)[(Ni_(1/3)Mn_(1/3)Co_(1/3))_(1−x)]O₂ prepared similar as inExample 1 with x=0.06 and firing at 900° C. No improvement in thecycling stability at 50° C. was observed between the two samples. ThisExample shows that there is a narrow composition range in which excesslithium and firing at 850° C. to 925° C. produces high voltage positiveelectrode materials.

Comparative Example 3 and Example 3

This example describes the synthesis ofLi_(1+x)[(Ni_(0.5)Mn_(0.3)Co_(0.2))_(1−x)]O₂ x=0.05 fired at 1000° C.(Comparative Example 3), and ofLi_(1+x)[(Ni_(0.5)Mn_(0.3)Co_(0.2))_(1−X)]O₂ x=0.06 fired at 900° C.(Example 3) following the procedures from Example 2 and ComparativeExample 2. Samples from Comparative Example 3 and Example 3 were cycledvs. lithium as described above. The results are displayed cycling isillustrated in FIG. 5.

FIG. 5 is a comparative graph of capacity (mAh/g) vs. cycle number forcompositions of Comparative Example 3 and Example 3. These results showthe importance of higher amounts of excess lithium and lower firingtemperature on the performance of the positive electrode compositions athigh voltages.

Comparative Example 4A and Comparative Example 4B

This example describes the synthesis according to Example 2 ofLi_(1+x)[(Ni_(0.17)Mn_(0.17)Co_(0.66))_(1−x)]O₂ x=0.05 fired at 1000° C.(Comparative Example 4A), and ofLi_(1+x)[(Ni_(0.17)Mn_(0.17)Co_(0.66))_(1−x)]O₂ x=0.06 fired at 900° C.(Comparative Example 4B). Samples from Comparative Examples 4A and 4Bwere cycled at 50° C. vs. lithium as described above. Both ComparativeExamples 4A and 4B displayed poor cycling at 50° C. with less than 90%capacity retention after cycle 52 as compared with the capacity aftercycle 2.

FIG. 6 is a comparative graph of capacity (mAh/g) vs. cycle number forcompositions of Comparative Example 4A (fired at 1000° C.) and 4B (firedat 900° C.) cycled at 50° C. Comparative Example 4A and 4B do not meetthe composition requirement of c/(a+b)<0.25, and both materials resultedin capacity retention of less than 90% over 50 cycles.

Comparative Example 5

This example describes the synthesis according to Example 2 ofLi_(1+x)[(Ni_(0.6)Mn_(0.3)Co_(0.1))_(1−x)]O₂ x=0.06 fired at 900° C. Theprecursor hydroxide could not be fired at 1000° C. with any meaningfulresults. The sample fired at 900° C. was observed to display an “oxygenloss” plateau. This comparative example shows that an “oxygen loss”plateau in the capacity vs. cell voltage curve is necessary but notsufficient for the production of stable high voltage positiveelectrodes. Comparative Example 5 does not meet the compositionrequirements of 0.6≦b/a≦1.2 and Comparative Example 5 displayed lessthan 90% capacity retention after 50 cycles.

Example 6

In a 10 L stirred tank reactor equipped with inlet and outlet ports,temperature control, controlled stirrer speed and pH probe control, wasadded 180 g of (Ni_(0.42)Mn_(0.42)Co_(0.16))(OH)₂ (same as above) and0.2M NH₃OH in 1.5 L deionized water. The dispersion was purged withargon under stirring and heated to 60° C. A 1.5M solution of Ni(SO₄).H₂Oand Mn(SO₄).H₂O (Ni/Mn ratio 0.44/0.56) was metered into the dispersionat a rate of 3 ml/min for 5 hrs to form a core shell transition metalhydroxide. The amount of transition metal sulfate added was sufficientto form a 30% (atomic ratio) shell. The hydroxide was washed and driedto form a transition metal hydroxide powder with a shell composition ofNi:Mn (44:56 Atomic ratio).

15 g of this hydroxide was mixed with 7.8258 g Li(OH).H₂O in a mortarand the mixture sintered at 500° C. for 4 hrs, then fired at 900° C. for12 hrs to form a target composition ofLi_(1+x){[(Ni_(0.42)Mn_(0.42)Co_(0.16))_(1−x)]_(0.70)[(Ni_(0.44)Mn_(0.56))_(1−x)]_(0.30)}O₂,with x=0.06.

FIG. 7 is a graph of capacity (mAh/g) vs. cycle number which illustratesthe improved cycling at 50° C. of Example 6 compared to Example 1, andCommercial Material A (fired at 1000° C.) and cycled at 50° C. FIG. 7demonstrates the added effect of providing a core shell composition.

Following are exemplary embodiments of high capacity positive electrodesfor use in lithium-ion electrochemical cells and methods of making sameaccording to aspects of the present invention.

Embodiment 1 is a positive electrode for a lithium-ion electrochemicalcell comprising a composition having the formula,

Li_(1+x)(Ni_(a)Mn_(b)Co_(c))_(1−x)O₂,

wherein 0.05≦x≦0.10, a+b+c=1, 0.6≦b/a≦1.1, c/(a+b)<0.25, and a, b, and care all greater than zero, wherein said composition has a capacityretention of greater than about 95% after 50 cycles when comparing thecapacity after cycle 52 with the capacity after cycle 2 when cycledbetween 2.5 V and 4.7 V vs. Li/Li⁺ at 30° C.

Embodiment 2 is a positive electrode for a lithium-ion electrochemicalcell according to embodiment 1, wherein 0.10≦c≦0.20.

Embodiment 3 is a positive electrode for a lithium-ion electrochemicalcell according to embodiment 2, wherein b/a is about 1.

Embodiment 4 is a positive electrode for a lithium-ion electrochemicalcell according to embodiment 1, wherein 0.05≦x≦0.07.

Embodiment 5 is a positive electrode for a lithium-ion electrochemicalcell according to embodiment 1, wherein the composition has beenprepared by heating to a temperature ranging from 850° C. to 925° C.

Embodiment 6 is a positive electrode for a lithium-ion electrochemicalcell according to embodiment 1, wherein said composition has a capacityretention of greater than about 90% after 50 cycles after 50 cycles whencomparing the capacity after cycle 52 with the capacity after cycle 2when cycled between 2.5 V and 4.7 V vs. Li/Li+ at 50° C.

Embodiment 7 is a lithium-ion electrochemical cell comprising: an anode;an electrolyte; and a positive electrode according to embodiment 1.

Embodiment 8 is a positive electrode for a lithium-ion electrochemicalcell comprising a composition that comprises a plurality of particlescomprising: a core having the formula,

Li_(1+x)(Ni_(a)Mn_(b)Co_(c))_(1−x)O₂,

wherein 0.05≦x≦0.10, a+b+c=1, 0.6≦b/a≦1.1, c/(a+b)<0.25, a, b, and c areall greater than zero; and a shell substantially surrounding the corecomprising a lithium mixed transition metal oxide comprising manganeseand nickel wherein the molar ratio of manganese to nickel is greaterthan b/a and b/a>1, wherein said composition has a capacity retention ofgreater than about 95% after 50 cycles when comparing the capacity aftercycle 52 with the capacity after cycle 2 when cycled between 2.5 V and4.7 V vs. Li/Li⁺ at 30° C.

Embodiment 9 is a positive electrode for a lithium-ion electrochemicalcell according to embodiment 8, wherein the core has a formula wherein0.10≦c≦0.20.

Embodiment 10 is a positive electrode for a lithium-ion electrochemicalcell according to embodiment 8, wherein the core has a formula wherein0.05≦x≦0.07.

Embodiment 11 is a positive electrode for a lithium-ion electrochemicalcell according to embodiment 8, wherein the composition has beenprepared by heating to a temperature ranging from 850° C. to 925° C.

Embodiment 12 is a positive electrode for a lithium-ion electrochemicalcell according to embodiment 8, wherein said composition has a capacityretention of greater than about 90% after 50 cycles after 50 cycles whencomparing the capacity after cycle 52 with the capacity after cycle 2when cycled between 2.5 V and 4.7 V vs. Li/Li⁺ at 50° C.

Embodiment 13 is a positive electrode for a lithium-ion electrochemicalcell according to embodiment 8, wherein the core has a formula,Li_(1.06)[Ni_(0.42)Mn_(0.42)Co_(0.16)]O₂ and the shell has a ratio ofb/a of 1.27.

Embodiment 14 is a lithium-ion electrochemical cell comprising: ananode; an electrolyte; and a positive electrode according to embodiment8.

Embodiment 15 is a method of making a positive electrode having theformula,

Li_(1+x)(Ni_(a)Mn_(b)Co_(c))_(1−x)O₂,

wherein 0.05≦x≦0.10, a+b+c=1, 0.6≦b/a≦1.1, c/(a+b)≦0.25 and a, b, and care greater than zero comprising: precipitating a transition metalhydroxide or carbonate with a molar ratio of Ni:Mn:Co of a:b:c mixingsaid transition metal hydroxide or carbonate with a Li source in a molarratio of Li to transition metal of [(1+x)/(1−x)] to 1; sintering themixture at about 500° C. for at least about 4 hours; and firing themixture at from about 850° C. to about 925° C. for at least 12 hoursafter sintering.

Embodiment 16 is a method of making a positive electrode according toembodiment 15, wherein 0.10≦c≦0.20.

Embodiment 17 is a method of making a positive electrode according toembodiment 15, wherein b/a is about 1.

Embodiment 18 is a method of making a positive electrode according toembodiment 15, wherein 0.05≦x≦0.07.

Embodiment 19 is a method of making a positive electrode according toembodiment 15, wherein said composition has a capacity retention ofgreater than about 90% after 50 cycles after 50 cycles when comparingthe capacity after cycle 52 with the capacity after cycle 2 when cycledbetween 2.5 V and 4.7 V vs. Li/Li⁺ at 50° C.

Embodiment 20 is a method of making a positive electrode comprising:precipitating a transition metal hydroxide or carbonate with a molarratio of Ni:Mn:Co of a:b:c with respect to the formula

Li_(1+x)(Ni_(a)Mn_(b)Co_(c))_(1−x)O₂,

wherein 0.05≦x≦0.10, a+b+c=1, 0.6≦b/a≦1.1, c/(a+b)≦0.25 and a, b, and care all greater than zero to form a hydroxide mixture; dispersing thetransition metal hydroxide or carbonate in ammoniated water; heating themixture to greater than about 60° C.; adding an aqueous solution ofsoluble mixed transition metal salts comprising manganese and nickelwherein the molar ratio of manganese to nickel is greater >1 andprecipitating to form a core-shell hydroxide or carbonate; drying thecore-shell hydroxide or carbonate;

mixing the core-shell hydroxide or carbonate with a lithium salt;

sintering the mixture at about 500° C. for at least about 4 hours; and

firing the mixture at from about 850° C. to about 925° C. for at least12 hours after sintering.

Various modifications and alterations to this invention will becomeapparent to those skilled in the art without departing from the scopeand spirit of this invention. It should be understood that thisinvention is not intended to be unduly limited by the illustrativeembodiments and examples set forth herein and that such examples andembodiments are presented by way of example only with the scope of theinvention intended to be limited only by the claims set forth herein asfollows. All references cited in this disclosure are herein incorporatedby reference in their entirety.

What is claimed is:
 1. A positive electrode for a lithium-ionelectrochemical cell comprising a composition having the formula,Li_(1+x)(Ni_(a)Mn_(b)Co_(c))_(1−x)O₂, wherein 0.05≦x≦0.10, a+b+c=1,0.6≦b/a≦1.1, c/(a+b)<0.25, and a, b, and c are all greater than zero,wherein said composition has a capacity retention of greater than about95% after 50 cycles when comparing the capacity after cycle 52 with thecapacity after cycle 2 when cycled between 2.5 V and 4.7 V vs. Li/Li⁺ at30° C.
 2. A positive electrode for a lithium-ion electrochemical cellaccording to claim 1, wherein 0.10≦c≦0.20.
 3. A positive electrode for alithium-ion electrochemical cell according to claim 2, wherein b/a isabout
 1. 4. A positive electrode for a lithium-ion electrochemical cellaccording to claim 1, wherein 0.05≦x≦0.07.
 5. A positive electrode for alithium-ion electrochemical cell according to claim 1, wherein thecomposition has been prepared by heating to a temperature ranging from850° C. to 925° C.
 6. A positive electrode for a lithium-ionelectrochemical cell according to claim 1, wherein said composition hasa capacity retention of greater than about 90% after 50 cycles after 50cycles when comparing the capacity after cycle 52 with the capacityafter cycle 2 when cycled between 2.5 V and 4.7 V vs. Li/Li+ at 50° C.7. A lithium-ion electrochemical cell comprising: an anode; anelectrolyte; and a positive electrode according to claim 1.